1. Technical Field
The present disclosure relates to methods for preparing and purifying electrolytes and electrolyte solutions useful in reduction-oxidation (redox) flow batteries.
2. Discussion
There is an increasing demand for novel and innovative electric power storage systems. Redox batteries have become an attractive means for such energy storage. In certain applications, a redox flow battery or a redox flow cell may include positive and negative electrodes disposed in separate half-cell compartments. The two half-cells may be separated by a porous or ion-selective membrane, through which ions are transferred during a redox reaction. Electrolytes (anolyte and catholyte) are flowed through the half-cells as the redox reaction occurs, often with an external pumping system. In this manner, the membrane in a flow cell battery operates in an aqueous electrolyte environment. In some applications, an iron-ion containing aqueous hydrochloric acid solution may be used as the catholyte, while a chromium-ion containing aqueous hydrochloric acid solution may be used as the anolyte. In some applications, a mixture of chromium and iron containing solutions may be used on both sides of the redox cell. The use of mixed reactants eliminates the requirement for a highly-selective membrane since the composition of both half cells is identical in the discharged state.
In some redox flow batteries, certain metal impurities contained in the electrolyte solution can cause side reactions at the negative electrode, which can result in the evolution of hydrogen gas that adversely affects the coulombic efficiency of the battery. While the use of high-purity raw materials such as high-grade iron chloride and high-grade chromium chloride can suppress such hydrogen gas-forming reactions, such materials are typically too expensive for use in redox batteries on a commercial scale.
Previously reported method of purification of electrolyte solution involves the preferential plating of nickel metal on an electrode of a flow battery utilizing lead to provide the hydrogen overpotential and aid the process. See Wakabayashi, A.; Umehara, Y.; Morie, S.; Kuwahara, “Process for the Preparation of Redox Battery Electrolyte and Recovery of Lead Chloride,” U.S. Pat. No. 4,875,483. However, considerable hydrogen production is obtained and the nickel plating process does not go to conclusion, resulting in a remaining high level of impurities in the electrolyte.
Therefore, there exists a need to develop methods for preparing and purifying electrolyte solutions from inexpensive raw materials. In addition, there exists a need to develop methods of removing impurities from those electrolyte solutions, and, in particular, those impurities associated with the evolution of hydrogen gas and decreased efficiency of the flow battery.